Rashes query. Them. attaching pictures.

My 4y 6m old son has developed rashes in his thighs (inner and outer) around 10 days back... after he tried on a new trouser. I took it off and washed it well and he hasn't worn it since then.. But even after 10-12 days, these rashes are not subsideing, but gradually spreading. Started from one thigh, moved on to the other thigh and now in one of the arms. These do not itch at all, and are not very pink or red. They are in clusters and not very spread out... the clusters are big and small here and there. Nothing on back or tummy. Kindly guide, as I am not able to understand if it is prickly heat or allergic reaction to the trousers or something else. Currently applying Candid powder on them. Attaching pictures.